?Quantize
While I try not to use quantize much when really recording, I find that my playing isn't always that tight when I'm experimenting and improvising. (It's easier to play tightly when you know what note you are going to play next!). So when I'm just beating on keys looking for a melody, it may not be played that well. When I'm writing, I'll just double click on the midi track I just recorded, highly the part(s) I want to keep and press the Quantize button. (You have to select the right quantize resolution or quantizing can make the playing worse..WAY WORSE!).

Keep in mind that quantizing will make the playing perfect...sometimes a little too perfect. Some people consider quantizing to sound "stiff". I'm not a big fan of "stiff" playing myself. Just keep in mind that I do consider the Quantize feature to be an extreme benefit when putting together a song. When the real deal tracking of the song is done, the results will be quite different.

You Can Build Up An Entire Song By Yourself
One mega advantage and disadvantage of using midi is that you don't need any help to make a song. You can create tremendous sounding drums with a mouse or a midi controller / keyboard. To get started with midi drum sequencing doesn't take long. To make the drums sound like a real drummer is hard work, but in the middle of a songwriting session, the basics do just fine.

I like to start with an acoustic guitar (if the song uses it) and play to a click track. After that, I go in and start programming the drums. It's not difficult to have have a rough drum track in 5 minutes or so. Midi allows you to cut and paste infinitely. (Again, not recommended when seriously tracking). However, when you are creating, there is nothing better than programming a few bars of drums, and then pasting them througout the verse and then pasting that verse throughout the song. Do the same with the chorus and the bridge and you are home free.

After the drums are programmed, I move on to the bass. I personally like using bass samples. Why? Because they sound amazing and if I decide to change the tempo of the song, it's very easy to get samples to adjust to what I'm looking for. (Remember, all I have to do is just the the bpm number in a little box and the entire song will adjust instantly if I'm using midi). From there, you could do a rough vocal take and be done with your song demo! A guy who is really good can do this entire process in 30-60 minutes.

If you wanted to really "finish" the song, you could various other instruments to make the song come to life. You'd want to adjust the drums to sound more human and then do the same with the bass. You might need to retrack the acoustic guitar to fit your new drums. But, you can see that it's totally possible and quite reasonable for one person to finish an entire song with full production by himself in the time it would take to setup and tune a drumset.